As has been explained by A. G. Evans, F. W. Zok, and J. Davis in "The Role of Interfaces in Fiber Reinforced Brittle Matrix Composites", Composite Science and Technology, 42, (1991) page 3, fiber reinforced glass-ceramic composites or any other ceramic matrix composite must have a weak interfacial bond between the fiber and the matrix material in order to assure the development of crack stopping, tough fracture behavior therein. Thus, in ceramic matrix composites demonstrating superior toughness, cracks which are generally initiated in the matrix are deflected along the interfacial boundary as fibers are debonded from the matrix. This necessary debonding will occur only if the debond energy is sufficiently low, when compared to the energy required to propagate a crack through the fiber, to facilitate crack deflection.
The production of SiC fiber reinforced, glass-ceramic composite articles has customarily involved three general steps:
(a) SiC fibers are coated with finely-divided particles of glass, which glass is a precursor of the desired glass-ceramic;
(b) the coated fibers are formed into tapes or woven into fabric layers which can then be stacked or otherwise fashioned into an article of a desired configuration; and thereafter
(c) that article is consolidated through heat and pressure into a composite article of high density.
Consolidation temperatures ranging over the 850.degree.-1400.degree. C. interval can be employed, with temperatures of at least 900.degree. C. being preferred.
Until recently, silicon carbide fiber reinforced, ceramic matrix composites that have satisfied the interfacial debonding requirement have largely had carbon- or boron nitride-dominated interfaces. Those interfaces are plagued with the problem of being easily oxidized once high temperature air reaches the interface through cracks (or porosity or other channels) from the outer surfaces of the composite. This oxidation proceeds rapidly, thereby effecting stronger fiber-matrix bonds. These stronger bonds inhibit debonding of the fibers with the result that, when stressed to develop cracks, those cracks run from the matrix through the fibers. This oxidation of the interfaces causes a precipitous loss of toughness of the composite, which phenomenon has been termed "oxidation embrittlement".
Therefore, there has been a need to develop functional debonding interfaces for SiC fiber reinforced, ceramic matrix composites which are resistant to oxidation and/or which modify the matrix material to eliminate the matrix cracking which creates pathways for the oxidation to take place over the desired range of temperatures and stress-strain behaviors. The primary objective of the present invention was to satisfy that need.